1. Field of Invention
This invention relates to a semiconductor process and a product thereof, and particularly relates to a method for fabricating a semiconductor device, and to a semiconductor device made by the method.
2. Description of Related Art
In certain types of semiconductor process, a SiGe layer is formed in a cavity in a silicon-substrate to, for example, induce a strain in an adjacent channel of a field-effect device and thereby improve the speed and so on of the device.
For example, as shown in FIG. 1, the field-effect device may be a PMOS device, of which the gate 102 is defined by a hard mask layer 104 on the substrate 100, an S/D extension 106 is formed with the gate 102 as a mask, and a spacer 108 is formed on the sidewall of the gate 102. A SiGe buffer layer 110 having a low Ge content is formed in the substrate 100 beside the gate 102 under a low pressure of about 20 torr, followed by a SiGe layer 112 having a high Ge content for inducing a sufficient strain.
However, the buffer layer 110 formed under low pressure is quite non-conformal, and is quite thin at the lower sidewall. Specifically, the ratio of the thickness of the buffer layer at the lower sidewall to the thickness of the buffer layer at the bottom is about 0.2. Hence, if a dislocation occurs in the SiGe layer 112, the buffer layer 110 at the lower sidewall becomes a weak point through which a Si(111) surface 120 breaks. If the buffer layer 110 at the lower sidewall is formed thicker to avoid such breaking through, the bottom part of the buffer layer 110 becomes overly thick so that the SiGe layer 112 is not deep enough to induce a sufficient strain.